U.S. patent application number 12/159946 was filed with the patent office on 2009-06-25 for apparatus for generating down link signal, and method and apparatus for cell search in cellular system.
This patent application is currently assigned to Electronics and Telecommunications Research Institute. Invention is credited to Seung-Chan Bang, Kap-Seok Chang, Il-Gyu Kim, Nam-Il Kim, Young-Hoon Kim.
Application Number | 20090161652 12/159946 |
Document ID | / |
Family ID | 39136071 |
Filed Date | 2009-06-25 |
United States Patent
Application |
20090161652 |
Kind Code |
A1 |
Chang; Kap-Seok ; et
al. |
June 25, 2009 |
APPARATUS FOR GENERATING DOWN LINK SIGNAL, AND METHOD AND APPARATUS
FOR CELL SEARCH IN CELLULAR SYSTEM
Abstract
The invention provides a method of generating a downlink signal
and searching a cell on the basis of the downlink signal in an
OFDM-based cellular system. The downlink signal includes a
plurality of synchronization blocks each having a plurality of
sub-frames, and a synchronization pattern composed of a combination
of a cell group identification code for identifying a cell group
and a frame synchronization identification code for indicating a
frame start point is generated in each of the synchronization
blocks. Different frame synchronization identification codes are
allocated to the synchronization blocks.
Inventors: |
Chang; Kap-Seok; (Daejeon,
KR) ; Kim; Il-Gyu; (Daejeon, KR) ; Kim;
Nam-Il; (Daejeon, KR) ; Kim; Young-Hoon;
(Daejeon, KR) ; Bang; Seung-Chan; (Daejeon,
KR) |
Correspondence
Address: |
CANTOR COLBURN, LLP
20 Church Street, 22nd Floor
Hartford
CT
06103
US
|
Assignee: |
Electronics and Telecommunications
Research Institute
Daejeon
KR
|
Family ID: |
39136071 |
Appl. No.: |
12/159946 |
Filed: |
May 25, 2007 |
PCT Filed: |
May 25, 2007 |
PCT NO: |
PCT/KR07/02556 |
371 Date: |
July 2, 2008 |
Current U.S.
Class: |
370/350 ;
375/260 |
Current CPC
Class: |
H04J 11/0079 20130101;
H04L 27/2613 20130101; H04L 27/2675 20130101; H04L 27/2656
20130101 |
Class at
Publication: |
370/350 ;
375/260 |
International
Class: |
H04J 3/06 20060101
H04J003/06; H04L 27/28 20060101 H04L027/28 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 28, 2006 |
KR |
10-2006-0081763 |
Claims
1-21. (canceled)
22. A method for generating a downlink frame including a
synchronization channel symbol duration, comprising: generating a
synchronization pattern by combination of a first code and a second
code; and allocating the synchronization pattern to a frequency
domain of the synchronization channel symbol duration to generate a
synchronization signal.
23. The method of claim 22, wherein the first code is different
from the second code.
24. The method of claim 22, wherein cell group information and
frame synchronization information are determined based on the
combination of the first code and the second code.
25. The method of claim 22, wherein one downlink frame includes a
plurality of synchronization channel symbol durations.
26. The method of claim 25, wherein a synchronization pattern
included in a first synchronization channel symbol duration within
a downlink frame duration is different from a synchronization
pattern included in a second synchronization channel symbol
duration within the same downlink frame duration.
27. The method of claim 22, wherein the first code is allocated to
even-numbered subcarriers in the frequency domain, and the second
code is allocated to odd-numbered subcarriers in the frequency
domain.
28. The method of claim 22, wherein a length of the first code is
identical to a length of the second code.
29. An apparatus for generating a downlink frame in a wireless
communication system, comprising: a pattern generator for
generating a synchronization pattern by combination of a first code
and a second code; and a mapping unit for mapping the
synchronization pattern generated by the pattern generator to a
frequency domain.
30. The apparatus of claim 29, wherein the first code is different
from the second code.
31. The apparatus of claim 29, wherein cell group information and
frame synchronization information are determined based on the
combination of the first code and the second code.
32. The apparatus of claim 29, wherein the synchronization pattern
is allocated to a synchronization channel symbol duration, wherein
one downlink frame includes a plurality of synchronization channel
symbol durations.
33. The apparatus of claim 32, wherein a synchronization pattern
included in a first synchronization channel symbol duration within
a downlink frame duration is different from a synchronization
pattern included in a second synchronization channel symbol
duration within the same downlink frame duration.
34. The apparatus of claim 29, wherein the first code is allocated
to even-numbered subcarriers in the frequency domain, and the
second code is allocated to odd-numbered subcarriers in the
frequency domain.
35. The apparatus of claim 29, wherein a length of the first code
is identical to a length of the second code.
36. A method for searching a cell, comprising: receiving a
synchronization pattern formed by combination of a first code and a
second code; and identifying cell group information and frame
synchronization information based on the combination of the first
code and the second code.
37. The method of claim 36, wherein the synchronization pattern is
allocated in a synchronization channel symbol duration, and one
frame includes a plurality of synchronization channel symbol
durations.
38. The method of claim 36, wherein the first code is allocated to
even-numbered subcarriers in the frequency domain, and the second
code is allocated to odd-numbered subcarriers in the frequency
domain.
39. An apparatus for searching a cell, comprising: a receiver for
receiving a synchronization pattern formed by combination of a
first code and a second code; and a cell group estimator for
estimating cell group information and frame synchronization
information based on the combination of the first code and the
second code.
40. The apparatus of claim 39, wherein the synchronization pattern
is allocated to a synchronization channel symbol duration, and one
frame includes a plurality of synchronization channel symbol
durations.
41. The apparatus of claim 39, wherein the first code is allocated
to even-numbered subcarriers in the frequency domain, and the
second code is allocated to odd-numbered subcarriers in the
frequency domain.
42. A computer-readable medium that stores a program that is
executable by a computer to perform a method comprising: generating
a synchronization pattern by combination of a first code and a
second code; and allocating the synchronization pattern to a
frequency domain of a synchronization channel symbol duration to
generate a synchronization signal.
43. The computer-readable medium of claim 42, wherein the first
code is different from the second code.
44. The computer-readable medium of claim 42, wherein cell group
information and frame synchronization information are determined
based on the combination of the first code and the second code.
45. The computer-readable medium of claim 42, wherein one frame
includes a plurality of synchronization channel symbol
durations.
46. The computer-readable medium of claim 45, wherein a
synchronization pattern included in a first synchronization channel
symbol duration within a frame duration is different from a
synchronization pattern included in a second synchronization
channel symbol duration within the same frame duration.
Description
TECHNICAL FIELD
[0001] The present invention relates to a method and apparatus for
generating a downlink signal in a cellular system, and more
particularly, to a method of searching a downlink cell in an
orthogonal frequency division multiplexing (OFDM)-based cellular
system.
BACKGROUND ART
[0002] In a cellular system, for initial synchronization, a
terminal should acquire timing synchronization and frequency
synchronization on the basis of signals transmitted from a base
station, and perform a cell search. After the initial
synchronization, the terminal should track the timing and
frequency, and perform the timing and frequency synchronization
between adjacent cells and the cell search in order for
handover.
[0003] In a synchronous cellular system, all base stations can
perform frame synchronization using common time information from an
external system. However, a cellular system that has been developed
by 3GPP (3rd generation partnership project) is an asynchronous
system in which the frame timings of all base stations are
independent. The asynchronous cellular system needs to perform a
cell search process, unlike the synchronous cellular system.
[0004] Therefore, a method of acquiring synchronization using a
separate preamble and searching a cell has been proposed. However,
the method cannot be applied to a system without the preamble. In
addition, a method of acquiring synchronization and searching a
cell using pilot symbols disposed at start and end points of a
sub-frame has been proposed. However, the method has a problem in
that a large number of pilots should be used.
DISCLOSURE
Technical Problem
[0005] The present invention has been made in an effort to provide
a cell searching method and apparatus that are capable of forming a
plurality of synchronization channels in one frame to effectively
acquire synchronization and search a cell in an OFDM-based cellular
system.
Technical Solution
[0006] In order to achieve the object, according to an exemplary
embodiment of the present invention, there is provided an apparatus
for generating a downlink signal in an orthogonal frequency
division multiplexing (OFDM)-based cellular system. The downlink
signal generating apparatus includes a pattern generator and a
time-frequency mapping unit. The pattern generator generates
synchronization patterns for a plurality of synchronization blocks
forming one frame of the downlink signal, and the synchronization
blocks each have a continuous series of sub-frames. The
synchronization pattern includes a cell group number and
information on a start point of the frame. The time-frequency
mapping unit maps the synchronization patterns to a time-frequency
domain to generate the downlink signal.
[0007] According to another exemplary embodiment of the present
invention, there is provided an apparatus for searching a cell
including a terminal in an orthogonal frequency division
multiplexing (OFDM)-based cellular system. The cell searching
apparatus includes a receiver and first to third estimators. The
receiver receives one frame of synchronization blocks. Each of the
synchronization blocks has a plurality of adjacent sub-frames, and
a plurality of OFDM symbols of the synchronization block each have
a synchronization pattern that is composed of a combination of a
cell group identification code for identifying a cell group and a
frame synchronization identification code for indicating a frame
start point. The combination of the cell group identification code
and the frame synchronization identification code is referred to as
a combination of codes. The first estimator estimates a start point
of the synchronization block from the synchronization pattern. The
second estimator estimates the frame start point and a cell group
number of the cell group to which the cell including the terminal
belongs, using the start point of the synchronization block. The
third estimator estimates a cell number of the cell including the
terminal, using a cell identification scrambling code included in a
pilot symbol of the frame.
[0008] According to still another exemplary embodiment of the
invention, there is provided a method of searching a cell including
a terminal in an orthogonal frequency division multiplexing
(OFDM)-based cellular system. First, a downlink frame including a
plurality of synchronization blocks, each having a synchronization
pattern that is composed of a combination of a cell group
identification code for identifying a cell group including the
terminal and a frame synchronization identification code for
indicating a start portion of the frame (a combination of codes),
is received, and a start point of the synchronization block is
estimated in the received downlink frame. Then, a cell group number
and frame synchronization are acquired from the estimated start
point of the synchronization block and the synchronization pattern,
and a cell number is acquired from a cell identification scrambling
code included in the downlink frame.
DESCRIPTION OF DRAWINGS
[0009] FIG. 1 is a block diagram schematically illustrating an
apparatus for generating a downlink signal in a cellular system
according to an exemplary embodiment of the present invention.
[0010] FIG. 2 is a diagram illustrating the configuration of a
downlink frame of the cellular system according to the exemplary
embodiment of the present invention.
[0011] FIG. 3 is a diagram illustrating the detailed configuration
of the downlink frame shown in FIG. 2.
[0012] FIG. 4 is a diagram illustrating a signal waveform obtained
by converting the downlink frame shown in FIG. 3 into a time
domain.
[0013] FIG. 5 is a diagram illustrating the bandwidth scalability
of the downlink frame according to the exemplary embodiment of the
present invention.
[0014] FIG. 6 is a diagram illustrating the bandwidth scalability
of a downlink frame according to another exemplary embodiment of
the present invention.
[0015] FIG. 7 is a block diagram schematically illustrating a cell
searching apparatus according to an exemplary embodiment of the
present invention.
[0016] FIG. 8 is a flowchart illustrating a cell searching method
according to an exemplary embodiment of the present invention.
[0017] FIG. 9 is a block diagram schematically illustrating the
configuration of a synchronization estimator according to an
exemplary embodiment of the present invention.
[0018] FIG. 10 is a diagram illustrating a method of allocating a
cell group identification code and a frame synchronization
identification code according to an exemplary embodiment of the
present invention.
[0019] FIG. 11 is a diagram illustrating a method of allocating a
cell group identification code and a frame synchronization
identification code according to another exemplary embodiment of
the present invention.
[0020] FIG. 12 is a block diagram schematically illustrating the
configuration of a cell group estimator according to an exemplary
embodiment of the present invention.
MODE FOR INVENTION
[0021] In the following detailed description, only certain
exemplary embodiments of the present invention have been shown and
described, simply by way of illustration. However, the present
invention is not limited to the following exemplary embodiments,
but various modifications and changes of the invention can be made.
Accordingly, the drawings and description are to be regarded as
illustrative in nature and not restrictive. Like reference numerals
designate like elements throughout the specification.
[0022] It will be understood that the terms "comprises" and/or
"comprising," when used in this specification, specify the presence
of stated features, integers, steps, operations, elements, and/or
components, but do not preclude the presence or addition of one or
more other features, integers, steps, operations, elements,
components, and/or groups thereof.
[0023] Hereinafter, a method and apparatus for generating a
downlink signal and a method and apparatus for searching a cell in
a cellular system according to exemplary embodiments of the present
invention will be described in detail with reference to the
accompanying drawings.
[0024] FIG. 1 is a block diagram schematically illustrating an
apparatus for generating a downlink signal in a cellular system
according to an exemplary embodiment of the present invention, and
FIG. 2 is a diagram illustrating a downlink frame structure of a
cellular system according to an exemplary embodiment of the present
invention.
[0025] As shown in FIG. 1, a downlink signal generating apparatus
100 according to an exemplary embodiment of the present invention
includes a pattern generator 110, a code generator 120, a
time-frequency mapping unit 130, an OFDM transmitter 141, and a
transmitting antenna 142, and is provided in a base station (not
shown) of the cellular system. As shown in FIG. 2, the downlink
signal generated by the downlink signal generating apparatus 100
according to the exemplary embodiment of the present invention
includes a plurality of synchronization blocks 210, and each of the
synchronization blocks 210 includes a plurality of sub-frames 220.
Information for identifying a cell group and information for
estimating frame synchronization are allocated to first symbol
durations 230a and 230b of each synchronization block 210. In
addition, different frame synchronization identification codes are
allocated to the synchronization blocks 210.
[0026] The pattern generator 110 generates a synchronization
pattern and a pilot pattern of the downlink signal using a set of
orthogonal codes indicating cell number information, cell group
information, and information for identifying frame synchronization.
The pattern generator 110 allocates a series of orthogonal codes to
a cell group number for identifying a cell group, and uses the
series of orthogonal codes to recognize a frame start point.
Hereinafter, for better comprehension and ease of description, the
orthogonal codes allocated to the cell group numbers are referred
to as "cell group identification codes," and the orthogonal codes
used to recognize the frame start points are referred to as "frame
synchronization identification codes." The pattern generator 120
matches the cell group identification codes with the frame
synchronization identification codes to generate a set of codes,
and allocates the set of codes to a frequency domain of a
synchronization channel symbol duration of the downlink signal to
generate a synchronization pattern of the downlink signal. The
pattern generator 110 allocates to a pilot channel symbol duration
a unique scrambling code that is allocated to each cell in order to
encode a common pilot symbol and a data symbol in the cellular
system, thereby generating a pilot pattern of the downlink
signal.
[0027] The code generator 120 generates orthogonal code sets that
are used as the cell group identification codes and the frame
synchronization identification codes, and transmits the generated
orthogonal code sets to the pattern generator 110. Then, the
pattern generator 110 uses the orthogonal code sets to generate a
synchronization pattern and a pilot pattern.
[0028] The time-frequency mapping unit 130 maps data to a
time-frequency domain, using the synchronization pattern
information and the pilot pattern information generated by the
pattern generator 110, and frame structure information and
transmission traffic data that are transmitted from the outside, to
form a frame of downlink signals (reference numeral 200 in FIG.
2).
[0029] Then, the OFDM transmitter 141 receives the downlink signal
from the time-frequency mapping unit 130, and transmits the signal
through the transmitting antenna 142.
[0030] Referring to FIG. 2, one frame 200 of downlink signals in a
cellular system according to an exemplary embodiment of the present
invention is composed of Nsync synchronization blocks 210, and each
of the synchronization blocks 210 includes Nsub sub-frames 220. An
OFDM symbol duration 230a of the downlink signal uses Nt
subcarriers each having a frequency range of .DELTA.f. Pilot symbol
durations 240a to 240e, each having pilot data therein, are formed
in the headers of the sub-frames 220 forming one synchronization
block 210. A first sub-frame of the synchronization block 210 is
provided with synchronization symbol durations 230a and 230b each
having data including a cell group identification code and a frame
synchronization identification code arranged therein. The
synchronization symbol durations 230a and 230b may be disposed in a
first OFDM symbol duration of the first sub-frame or the last OFDM
symbol duration of the first sub-frame. Each of the synchronization
symbol durations 230a and 230b is divided into two frequency bands
250 and 260 in the frequency domain, and each of the frequency
bands 250 and 260 has the cell group identification code and the
synchronization identification code inserted therein. As shown in
FIG. 2, the pattern generator 110 does not form a synchronization
pattern in the entire frequency domain of each of the symbol
durations 230a and 230b, but allocates codes to only a central
portion of the frequency bandwidth except a DC subcarrier to form
the synchronization pattern in the central portion. In a 3GPP
system, the downlink frame 200 includes 20 sub-frames 220, and one
sub-frame 220 corresponds to a time of 0.5 msec. In the case of
unicast transmission, one sub-frame 220 includes 7 OFDM symbol
durations, and in the case of multicast transmission, one sub-frame
220 includes 6 OFDM symbol durations. In the downlink frame of the
3GPP system, as an example, the synchronization block 210 may
include 5 sub-frames 220. In this case, one frame includes four
synchronization channel symbol durations.
[0031] Next, the generation of the synchronization pattern and the
pilot pattern by the pattern generator 110 shown in FIG. 1 will be
described in detail with reference to FIGS. 3 and 4.
[0032] FIG. 3 is a diagram illustrating the OFDM symbols in the
synchronization channel symbol duration in which the
synchronization pattern is formed, and FIG. 4 is a diagram
illustrating a signal waveform when the synchronization channel
symbol duration shown in FIG. 3 is converted into a time
domain.
[0033] As shown in FIG. 3, the pattern generator 110 divides a
predetermined bandwidth into a frequency band 250 for inserting the
cell group identification code and a frequency band 260 for
inserting the frame synchronization identification code on the
basis of a central subcarrier in the entire frequency bandwidth of
the channel symbol duration 230a, and sequentially inserts
orthogonal codes into the divided frequency bands to form the
synchronization pattern.
[0034] The pattern generator 110 allocates to the frequency bands
250 and 260 the orthogonal codes in two independent orthogonal code
sets transmitted from the code generator 120. Referring to FIG. 3,
the pattern generator 110 allocates an orthogonal code set of and
an orthogonal code set of to the frequency band 250 for identifying
a cell group and the frequency band 260 for identifying frame
synchronization to form the synchronization pattern, respectively.
In this case, "k" indicates a cell group number, "u" indicates a
frame synchronization identification code number, "NG" indicates
the length of the cell group identification code, and "NF"
indicates the length of the frame synchronization identification
code. The pattern generator 110 according to the exemplary
embodiment of the present invention may use GCL (generalized
chirp-like) codes as the cell group identification code and the
frame synchronization identification code, and these codes can be
expressed by the following Equations 1 and 2:
c n ( k ) = exp { - j 2 .pi. k n ( n + 1 ) 2 N G } , n = 0 , 1 , ,
N G - 1 , and ( Equation 1 ) c n ( u ) = exp { - j2 .pi. u n ( n +
1 ) 2 N F } , n = 0 , 1 , , N F - 1. ( Equation 2 )
##EQU00001##
[0035] The orthogonal codes expressed by Equations 1 and Equation 2
are allocated to the positions shown in FIG. 3 to generate the
synchronization pattern. That is, the pattern generator 110 does
not sequentially allocate the orthogonal codes obtained by
Equations 1 and 2 to adjacent subcarriers, but allocates
even-numbered subcarriers or odd-numbered subcarriers in the
frequency bands 250 and 260. Subcarriers between the subcarriers
having the orthogonal codes allocated thereto are used as nulling
subcarriers to which no sequence is allocated. Therefore, the
subcarriers including the nulling carriers that are arranged in the
synchronization channel symbol duration for forming the pattern
occupy substantially 2*[(NG+NF)+NB] (hereinafter, referred to as
NS) subcarrier bands. In this case, "NB" indicates the number of
subcarriers in a guard band.
[0036] When the synchronization pattern is converted into a time
domain, the signal waveform shown in FIG. 4 is obtained. FIG. 4
shows the signal waveform of the OFDM symbol except a cyclic
prefix. As can be seen from FIG. 4, two repeated patterns are
generated in the time domain due to two kinds of inserted
orthogonal codes.
[0037] As shown in FIG. 3, the downlink signal generating apparatus
100 according to the exemplary embodiment of the present invention
forms a synchronization pattern such that one nulling subcarrier
exists between the subcarriers to which sequences are allocated
over the frequency domain of the synchronization channel symbol
duration in which the cell group identification code and the
synchronization identification code are allocated, thereby
generating signals. Therefore, the generated signal has the
repeated pattern shown in FIG. 4, and a terminal having received
the downlink frame acquires initial symbol synchronization and
estimates a frequency offset, using the signal pattern shown in
FIG. 4.
[0038] The lengths NG and NF of the cell group identification code
and the synchronization identification code inserted into each of
the synchronization channel symbol durations of the downlink frame
may be different from each other, and information on the lengths of
these identification codes and information on the synchronization
patterns thereof are shared by a terminal and a base station.
[0039] The terminal having received the downlink frame 200 having
the synchronization pattern shown in FIG. 3 demodulates the two
frequency bands 250 and 260 for each synchronization block to
obtain information on the cell group number and the frame start
point, which makes it possible to rapidly and effectively search
the cells. In addition, the frequency domain of the channel symbol
duration is divided into two frequency bands, and the same sequence
or different types of sequences are allocated to the two divided
frequency bands, which makes it possible to prevent the lowering of
a correlation performance due to the selective fading of
frequencies.
[0040] In the exemplary embodiment of the present invention, the
cell group identification code is inserted before the frame
synchronization identification code on a frequency axis of the
synchronization channel symbol duration, but the invention is not
limited thereto. For example, the cell group identification code
may be inserted after the frame synchronization identification code
to form the synchronization pattern. Further, in the exemplary
embodiment of the present invention, the same type of orthogonal
code is used as the cell group identification code and the frame
synchronization identification code, but the invention is not
limited thereto. For example, different types of orthogonal codes
may be used as the cell group identification code and the frame
synchronization identification code. In this case, general
orthogonal codes, such as a Hadamard code, a KAZAC code, a gold
code, a Golay code, and a pseudo-noise (PN) code, may be used as
the identification codes.
[0041] FIG. 5 is a diagram illustrating the bandwidth scalability
of a downlink frame according to an exemplary embodiment of the
present invention, and FIG. 6 is a diagram illustrating the
bandwidth scalability of a downlink frame according to another
exemplary embodiment of the present invention.
[0042] FIGS. 5 and 6 show the comparison between the bandwidth of
the synchronization channel symbol duration shown in FIG. 3 with
the entire bandwidth supported by the cellular system. As shown in
FIGS. 2 and 3, the downlink signal generating apparatus 100
according to the exemplary embodiment of the present invention
inserts orthogonal codes into the center of the frequency bandwidth
to generate a synchronization pattern. In the cellular system,
since the terminals have different supportable bandwidths according
to their levels, it is possible to support the bandwidth
scalability of the terminals through the frame structure. FIG. 5
shows a synchronization pattern allocated to a 1.25 MHz band within
the frequency bandwidth. Traffic data cannot be allocated to an
OFDM symbol without a synchronization pattern in the channel symbol
duration, and transmitted thereto. FIG. 6 shows a synchronization
pattern allocated to a 1.25 MHz band or a 5 MHz band within the
frequency bandwidth. A terminal supporting a 5 MHz band or more can
receive all synchronization patterns transmitted, but terminals
supporting a 1.25 MHz band and a 2.5 MHz band can receive some
synchronization patterns that are arranged in the center of the
frequency bandwidth. According to the exemplary embodiment of the
present invention, it is possible to extract the cell group number
and information on the synchronization start point from the
downlink frame using only some of the received synchronization
patterns, and thus support the bandwidth scalability.
[0043] Next, a method of allowing a terminal to search a cell using
the downlink signal will be described in detail below with
reference to FIGS. 7 and 8.
[0044] FIG. 7 is a block diagram schematically illustrating a cell
searching apparatus according to an exemplary embodiment of the
present invention, and FIG. 8 is a flowchart illustrating a cell
searching method according to an exemplary embodiment of the
present invention.
[0045] Referring to FIG. 7, a cell searching apparatus 400
according to an exemplary embodiment of the present invention
includes a receiver 410, a symbol synchronization estimator 420, a
Fourier transformer 430, a cell group estimator 440, and a cell
number estimator 450. The Fourier transformer 430 can perform fast
Fourier transform (FFT).
[0046] As shown in FIG. 8, the receiver 410 receives signals
transmitted from a base station. The symbol synchronization
estimator 420 filters the received signal within the bandwidth
allocated to a synchronization channel, removes a guard interval,
performs differential correlation to acquire symbol synchronization
or sub-frame synchronization, and estimates a frequency offset
(S110). Then, the Fourier transformer 430 performs Fourier
transform on the received signal on the basis of the symbol
synchronization estimated by the symbol synchronization estimator
420 (S120). The cell group estimator 440 estimates a frame start
point from the sequence of the synchronization channel symbol
duration included in the received signal that has been subjected to
Fourier transform, acquires frame synchronization, and estimates
the cell group number (S130). The cell number estimator 440
estimates the cell number using scrambling code information
included in the pilot symbol duration (S140).
[0047] Next, the acquisition of sub-frame synchronization and the
estimation of a frequency offset by the symbol synchronization
estimator 420 will be described in detail with reference to FIG.
9.
[0048] FIG. 9 is a block diagram schematically illustrating the
structure of the symbol synchronization estimator 420 according to
an exemplary embodiment of the present invention.
[0049] Referring to FIG. 9, the symbol synchronization estimator
420 according to the exemplary embodiment of the present invention
includes a filter 421, a delay unit 422, a correlator 423, a power
detector 424, a comparator 425, and a frequency offset detector
426.
[0050] The symbol synchronization estimator 420 estimates sub-frame
synchronization and frequency offset from a received signal having
the time domain signal waveform shown in FIG. 4 in the
synchronization channel symbol duration. The symbol synchronization
estimator 420 may estimate the last OFDM symbol duration of the
sub-frame where the synchronization pattern is formed and a
frequency offset in the last OFDM symbol duration.
[0051] The filter 421 filters the time domain signal within a
bandwidth allocated to the synchronization channel and removes a
guard interval to extract signals y(n+l) in NS subcarrier bands,
which are central subcarrier bands, in which the synchronization
patterns are formed in the entire frequency band corresponding to
the synchronization channel symbol duration. The filter 421 can
perform bandpass filtering. The length of the signal y(n+l) output
from the filter 421 corresponds to NS.
[0052] The delay unit 422 delays the filtered signal y(n+l) by a
time corresponding to half the effective symbol length NS. The
correlator 423 performs differential correlation on the input
signal y(n+l) and an output signal y(n+l+Ns/2) of the delay unit
422 in a sample duration corresponding to half the effective symbol
length. The differential correction performed by the correlator 423
can be expressed by Equation 3 given below:
Y = l = 0 1 / 2 N S - 1 y ( n + l ) y * ( n + l + 1 2 N S ) . (
Equation 3 ) ##EQU00002##
[0053] The power detector 424 having received the correlation
result Y calculated by Equation 3 calculates a differential
correlation value of the received signal, that is, the power of the
received signal. The comparator 425 selects the time when the power
detector 424 outputs a maximum value by Equation 4 given below, and
sets the selected time as an initial symbol synchronization
time.
.tau. ^ = max l { Y 2 } . ( Equation 4 ) ##EQU00003##
[0054] The frequency offset detector 426 estimates an initial
frequency offset.
[0055] In this exemplary embodiment of the present invention,
differential correlation is performed on only the time domain
signals corresponding to one synchronization channel symbol
duration to detect the initial symbol synchronization and the
frequency offset, but the invention is not limited thereto. For
example, the time domain signals in a different synchronization
channel symbol duration in one downlink frame may be accumulated,
and the differential correlation may be performed on the
accumulated signals. In addition, in order to improve an estimating
performance, data obtained from synchronization patterns of a
plurality of frames may be accumulated, and the differential
correlation may be performed on the accumulated data.
[0056] The Fourier transformer 430 performs Fourier transform on
the received signal on the basis of sub-frame synchronization
estimated by the symbol synchronization estimator 420.
[0057] The estimation of the frame synchronization and the cell
group number by the cell group estimator 440 from the
synchronization pattern of the signal that has been subjected to
Fourier transform will be described in detail below with reference
to FIGS. 10 to 12. First, referring to FIGS. 10 and 11, a method of
generating the synchronization pattern of the downlink frame and
estimating the cell group number and frame synchronization from the
generated synchronization pattern will be described with reference
to FIG. 12.
[0058] FIGS. 10 and 11 are diagrams illustrating a method of
allocating the synchronization pattern shown in FIG. 3. The
downlink generating apparatus according to the exemplary embodiment
of the present invention combines a cell group identification code
C(k) with a frame synchronization identification code C(u) to
generate a synchronization pattern. FIGS. 10 and 11 show
combinations of the cell group identification codes and the frame
synchronization identification codes in the form of (k, u) (A in
FIG. 10 and A' in FIG. 11). In FIGS. 10 and 11, it is assumed that
a frame 200 of downlink signals includes 4 synchronization blocks
210.
[0059] FIG. 10 shows a synchronization pattern generated by
combining orthogonal codes using only common frame synchronization
identification codes C(1), C(2), C(3), and C(4) to all cell groups
in the cellular system. In FIG. 10, cell No. 1 to cell No. 4 form
cell group No. 1, cell No. 5 to cell No. 8 form cell group No. 2,
and cell No. 9 to cell No. 12 form cell group No. 3. FIG. 10 shows
a combination of codes when C(k) (k is cell group number, k=1, 2,
3, . . . ) is used as the cell group identification code. When the
synchronization pattern is formed as shown in FIG. 10, the same
frame synchronization identification code is transmitted from all
cells. Therefore, it is possible to obtain a macro diversity gain.
That is, the terminal having received the downlink frame performs
correlation on a synchronization channel symbol duration to detect
a frame synchronization identification code, in order to acquire
the frame synchronization. In this case, since the same code is
used for all cells, a correlation characteristic is improved, and
thus a frame synchronization acquiring performance can be improved.
In this case, the number of cell groups that can be divided may be
set to be equal to the length of the code that is set to identify
the cell groups, and the length of the frame synchronization
identification code may be smaller than the length of the cell
group identification code due to the diversity gain.
[0060] FIG. 11 shows the formation of a frame 200 of downlink
signals using a combination of codes that is formed by allocating
different frame synchronization identification codes to the cell
groups. In this case, the number of frame synchronization
identification codes that are available in the cellular system is
equal to the length of the codes. When the synchronization pattern
is formed as shown in FIG. 11, the number of combinations of the
cell group numbers and the frame synchronization identification
codes increases since various frame synchronization identification
codes are used. Therefore, as compared with the synchronization
pattern shown in FIG. 10, it is possible to increase the number of
cell groups that can be identified.
[0061] A base station and terminals share information on the
combination of codes according to the exemplary embodiment of the
present invention, and the terminals use the information to search
cells.
[0062] FIG. 12 is a block diagram schematically illustrating the
cell group estimator 440 according to the exemplary embodiment of
the present invention.
[0063] As shown in FIG. 12, the cell group estimator 440 according
to the exemplary embodiment of the present invention includes a
code storage unit 441, a correlator 442, an inverse Fourier
transformer 443, and a comparator 444.
[0064] The code storage unit 441 stores orthogonal codes that are
used as the cell group identification codes and the frame
synchronization identification codes allocated to the
synchronization channel symbol duration, and also stores
information on the combination of codes forming the synchronization
pattern. Meanwhile, when information on the cell including a
terminal therein and peripheral cells (information on the cell
number and the cell group) is known beforehand (that is, when the
terminal is busy or in a standby state), the code storage unit 441
can extract a candidate combination of codes, and use the extracted
combination of codes to search cells.
[0065] The correlator 442 receives the signals in the
synchronization channel symbol duration that have been subjected to
Fourier transform, and multiplies the signals having been subjected
to Fourier transform by the conjugates of the orthogonal codes
included in a combination of codes that are stored in the code
storage unit 441.
[0066] That is, when the correlator 442 sequentially performs a
conjugate operation on sequences in the synchronization channel
section of the received downlink frame over the frequency domain,
an operation for identifying a cell group and an operation for
estimating frame synchronization are sequentially performed, which
makes it possible to shorten the time to search cells.
[0067] The inverse Fourier transformer 443 performs inverse Fourier
transform on a cell group identifying band and a frame
synchronization identifying band among the signals output from the
correlator 442 to generate time domain signals. In this case, the
inverse Fourier transformer 443 may perform inverse fast Fourier
transform (IFFT). The comparator 444 selects the maximum value from
the time domain signals output from the inverse Fourier transformer
443, and extracts information on a combination of codes having the
maximum value from the code storage unit 441, thereby identifying
the cell group number and the frame synchronization. As can be seen
from FIG. 10, as an example, when information on a combination of
codes extracted by the comparator 444 is (1, 2), the current cell
belongs to the cell group No. 1, and the terminal starts estimating
the frame synchronization in the second synchronization block of
the downlink frame. In this way, it is possible to estimate a frame
start point.
[0068] Finally, the terminal estimates the cell number using
scrambling information included in the pilot symbol duration. Since
the terminal knows the cell group information, the terminal
estimates the cell number on the basis of the scramble information
of the cells belonging to the corresponding cell group. In this
case, a general estimating method, such as a method of using the
sum of powers of a set of subcarriers of the pilot symbol, may be
used to estimate the cell number.
[0069] In this exemplary embodiment of the present invention, the
cell number is estimated from the scrambling information of the
pilot symbol duration, but the invention is not limited thereto.
For example, the cell number may be estimated by using symbols in a
common channel section including system information of a base
station.
[0070] In addition, in this exemplary embodiment of the present
invention, the cell group identification code is allocated to the
synchronization pattern, but the invention is not limited thereto.
Instead of the cell group identification code, a cell
identification code may be allocated to one of two bands of the
synchronization symbol duration to generate a downlink frame. In
this case, the estimation of the cell number using the scramble
code may be used to verify cell number information obtained from
the synchronization pattern.
[0071] The constituent elements according to the exemplary
embodiment of the present invention may be implemented by at least
one hardware component composed of a programmable logic element,
such as a DSP (digital signal process) processor, a controller, an
ASIC (application specific integrated circuit), or a FPGA (field
programmable gate array), other electronic devices, or a
combination thereof. In addition, at least a portion of the
function or procedure according to the exemplary embodiment of the
present invention may be executed by software, and the software may
be recorded on a recording medium. Further, the constituent
elements, the function, and the procedure according to the
exemplary embodiment of the present invention may be implemented by
a combination of hardware and software.
[0072] While this invention has been described in connection with
what is presently considered to be practical exemplary embodiments,
it is to be understood that the invention is not limited to the
disclosed embodiments, but, on the contrary, is intended to cover
various modifications and equivalent arrangements included within
the spirit and scope of the appended claims.
[0073] As described above, according to the exemplary embodiment of
the present invention, it is possible to use a plurality of
synchronization patterns formed in one frame to search a cell group
and to estimate frame synchronization. In addition, it is possible
to use the synchronization patterns to estimate sub-frame
synchronization.
* * * * *